This invention relates to a normally white (NW) liquid crystal display (LCD) including tilted and negative retardation films or retarders. More particularly, this invention relates to a NW twisted nematic (TN) LCD including a pair of tilted retarders arranged in a manner so as to provide improved contrast at vertical viewing angles and reduce gray level inversion.
This application is related to commonly owned U.S. Pat. Nos. 5,576,861; 5,594,568; and 5,570,214; and U.S. Ser. Nos. 08/559,275, filed Nov. 15, 1995; U.S. Ser. No. 08/711,797, filed Sep. 10, 1996, U.S. Ser. No. 08/768,502, filed Dec. 18, 1996, U.S. Ser. No. 08/869,973, filed Jun. 5, 1997, and U.S. Ser. No. 8/876,043, filed Jun. 13, 1997, the entire disclosures of which are all hereby incorporated herein by reference. Each of these commonly owned applications and/or patents relates to a liquid crystal display with specific retarder values, contrast ratios, retarder positions or orientations, and/or retarder types.
Informational data in liquid crystal displays (LCDs) is presented in the form of a matrix array of rows and columns of numerals or characters (e.g. pixels) which are generated by a number of segmented pixel electrodes arranged in a matrix pattern. The segments are connected by individual leads to driving electronics which apply a voltage to the appropriate combination of segments and adjacent liquid crystal (LC) material in order to display the desired data and/or information by controlling the light transmitted through the liquid crystal (LC) material.
Contrast ratio (CR) is one of the most important attributes considered in determining the quality of both normally white (NW) and normally black (NB) LCDS. The contrast ratio (CR) in a normally white display is determined in low ambient conditions by dividing the xe2x80x9coff-statexe2x80x9d light transmission (high intensity white light) by the xe2x80x9con-statexe2x80x9d or darkened transmitted intensity. For example, if the xe2x80x9coff-statexe2x80x9d transmission is 200 fL at a particular viewing angle and the xe2x80x9con-statexe2x80x9d transmission is 5 fL at the same viewing angle, then the display""s contrast ratio at that particular viewing angle is 40 (or 40:1) for the particular driving voltages utilized.
Accordingly, in normally white LCDs, a significant factor adversely limiting contrast ratio is the amount of light which leaks through the display in the darkened or xe2x80x9con-state.xe2x80x9d In a similar manner, in normally black displays, a significant factor limiting the contrast ratio achievable is the amount of light which leaks through the display in the darkened or xe2x80x9coff-state.xe2x80x9d The higher and more uniform the contrast ratio of a particular display over a wide range of viewing angles, the better the LCD in most applications.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristicsthan do their counterpart NW displays (i.e. the NB image can often be seen better at large or wide viewing angles). However, NB displays are optically different than NW displays and are much more difficult to manufacture due to their high dependence on the cell gap or thickness xe2x80x9cdxe2x80x9d of the liquid crystal layer as well as on the temperature of the liquid crystal (LC) material itself. Accordingly, a long-felt need in the art has been the ability to construct a normally white (NW) display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult and expensive to manufacture NB displays in order to achieve these characteristics.
What is often needed in NW LCDs is an optical compensating or retarding element(s), i.e. retardation film(s), which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be substantially blocked by the output polarizer (analyzer) in the xe2x80x9con-state.xe2x80x9d Optical compensating elements or retarders are known in the art and are disclosed, for example, in U.S. Pat. Nos. 5,184,236; 5,189,538; 5,406,396; 4,889,412; 5,344,916; 5,196,953;: 5,138,474; and 5,071,997.
The disclosures of U.S. Pat. Nos. 5,570,214 and 5,576,861 (incorporated herein by reference) in their respective xe2x80x9cBackgroundxe2x80x9d sections illustrate and discuss contrast ratio, and driving voltage versus intensity (fL) graphs of prior art NW displays which are less than desirable. Prior art NW LCD viewing characteristics are problematic in that, for example, their contrast ratios are limited horizontally and/or vertically (and are often non-symmetric), and their gray level performance lacks consistency.
Gray level performance, and the corresponding amount of inversion, are also important in determining the quality of an LCD. Conventional active matrix liquid crystal displays (AMLCDs) typically utilize anywhere from about 8 to 64 different driving voltages. These different driving voltages are generally referred to as xe2x80x9cgray levelxe2x80x9d voltages. The intensity of light transmitted through the pixel(s) or display depends upon the driving voltage utilized. Accordingly, conventional gray level voltages are used to generate dissimilar shades of color so as to create different colors and images when, for example, the shades are mixed with one another.
Preferably, the higher the driving voltage in a normally white display, the lower the intensity (fL) of light transmitted therethrough. The opposite is true in NB displays. Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB LCD to emit desired intensities and shades of light/color. A gray level voltage VON is generally known as any driving voltage greater than Vth (threshold voltage) up to about 3.0 to 6.5 volts, although gray level voltages may be as low as 2.0 in certain applications.
U.S. Pat. Nos. 5,576,861 and 5,570,214 discuss, in their respective xe2x80x9cBackgroundxe2x80x9d sections, prior art NW LCDs with inversion problems (e.g. inversion humps). As discussed therein, inversion humps are generally undesirable.
U.S. Pat. No. 5,583,679 discloses an LCD including an optical compensating sheet that includes a discotic structure and negative birefringence, with the discotic structure unit having an inclined plane. Unfortunately, the contrast ratios and inversion characteristics resulting from displays of the ""679 patent have been found by the instant inventors to be less than desirable. Certain embodiments of the instant invention described herein exhibit surprisingly improved results with respect to contrast ratio and/or inversion as compared to the ""679 patent.
In the prior art, some have been able to separately and independently reduce inversion or improve contrast in given viewing zones, but typically if contrast is improved upon, then inversion characteristics suffer. To date, those in the art have been unable to improve both contrast and inversion in the same viewing area of a display as taught below in accordance with the instant claimed invention.
The examples set forth in U.S. Application Ser. No. 08,876,043, incorporated herein be reference, have excellent viewing characteristics. However, it has been surprisingly found by the instant inventors that contrast ratios can be improved in the vertical viewing region(s) by the inventions set forth herein.
It is apparent from the above that there exists a need in the art for a normally white TN liquid crystal display wherein the viewing zone of the display has high contrast ratios and/or little or no inversion over a wide range of viewing angles, and wherein contrast ratios may be improved in vertical region(s). Furthermore, there exists a need in the art for improved contrast and reduced inversion in the same viewing zone (e.g. in the upper vertical viewing zone principally utilized by pilots of aircraft in avionic applications).
The term xe2x80x9crearxe2x80x9d when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing films or zones, and orientation films means that the described element is on the backlight side of the liquid crystal material, or in other words, on the side of the LC material opposite the viewer.
The term xe2x80x9cfrontxe2x80x9d when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing films or zones and orientation films means that the described element is located on the viewer side of the liquid crystal material.
Unless otherwise specified, the actual LCDs and/or light valves made and/or tested herein included a liquid crystal material with a birefringent value (xcex94n) of 0.0854 at room temperature, Model No. ZLI-4718 obtained from Merck.
Unless otherwise specified, the term xe2x80x9cretardation valuexe2x80x9d as used herein for uniaxial retarders means xe2x80x9cd.xcex94nxe2x80x9d of the retardation film or plate, where xe2x80x9cdxe2x80x9d is the film or plate thickness and xe2x80x9cxcex94nxe2x80x9d is the film birefringence (i.e. difference in certain indices of refraction).
For all circular contrast ratio graphs herein, xe2x80x9cEZContrastxe2x80x9d equipment available from Eldim of Caen, France (ID #204F) was used to develop these graphs. This equipment includes a system for measuring Luminance and Contrast versus viewing angle (incident (polar) and azimuth angle), utilizing 14 bits A/D conversion to give luminance measurements from {fraction (1/10)} to 8,000 cd/m2, with an accuracy of 3% and a fidelity of 1%. A temperature regulated CCD sensor with a photopic response (and specially designed lenses) are part of this commercially available Eldim system and corresponding software. The measurement device of this Eldim system includes a specially designed large viewing angle lens system having a numerical aperture of 0.86. The Eldim software is Windows(trademark) 3.1 based, running on any 486 and above PC, supporting DDE interface with other programs.
Unless otherwise specified, all examples herein were carried out at approximate 25xc2x0 C.
All measured real data herein, in the Examples, included the non-uniform characteristics of the backlight over a range of angles. Backlights are more intense at normal than at wide angles.
Generally speaking, this invention fulfills the above-described needs in the art by providing a method of making a normally white twisted nematic liquid crystal display comprising the steps of:
providing first and second negative tilted retarders whose tilt or incline angles vary in one direction through the thickness of the retarders;
providing a first negative non-tilted retarder;
disposing a twisted nematic liquid crystal layer between said first and second negative tilted retarders; and
orienting said tilted and non-tilted retarders so that the resulting display outputs a contrast ratio of at least about 80:1 at a 0xc2x0 vertical viewing angle over a horizontal angular span of at least about 50xc2x0, and a contrast ratio of at least about 50:1 at a viewing angle of 0xc2x0 horizontal and +40xc2x0 vertical.
This invention further fulfills the above-described needs in the art by providing a normally white twisted nematic liquid crystal display comprising:
a twisted nematic liquid crystal layer for twisting at least one normally incident wavelength of visible light from about 80xc2x0-100xc2x0 when in the off-state;
front and rear orientation means sandwiching said liquid crystal layer therebetween, said front orientation means including at least a front orientation direction and said rear orientation means including at least a rear orientation direction different than said front orientation direction;
first and second tilted retardation members located on opposite sides of said liquid crystal layer, said first tilted retardation member being located on the same side of said liquid crystal layer as said front orientation means, and said second tilted retardation member being located on the same side of said liquid crystal layer as said rear orientation means;
a first negative retardation member located on the rear side of said liquid crystal layer between said liquid crystal layer and said second tilted retardation member;
each of said first and second tilted retardation members having an optical axis defining an azimuthal angle, and a polar or inclined angle which varies through the thickness of the member; and
wherein said azimuthal angle of said first tilted retardation member is oriented parallel within about xc2x110xc2x0 relative to said front orientation direction so that said azimuthal angle of said first tilted retardation member and said front orientation direction are oriented in substantially the same direction, and said azimuthal angle of said second tilted retardation member is oriented parallel within about xc2x110xc2x0 relative to said rear orientation direction, and said azimuthal angle of said second tilted retardation member is parallel within xc2x110xc2x0 relative to a transmission axis of said rear polarizer.
In certain embodiments, the negative non-tilted retardation layer(s) being defined by one of: (i) nx greater than ny  greater than nz; and (ii) nx=ny greater than nz.
In certain embodiments, each tilted retarder has a retardation value d.(ne-no) of from about xe2x88x9220 to xe2x88x92200 nm, and preferably from about xe2x88x9250 to xe2x88x92150 nm, and most preferably from about xe2x88x92100 to xe2x88x92150 nm.
This invention will now be described with respect to certain embodiments thereof, along with reference to the accompanying illustrations, wherein: